1. Field of the Invention
This invention relates to electronic fuel injection system for engine and, more particularly, to an electronically controlled fuel injection system which can obtain the fuel injection timing without use of any exclusive sensor.
2. Description of the Prior Art
In a well-known electronic fuel injection system, a fuel injection valve is provided in an intake manifold of the engine, and its "on" or "open" time is controlled according to the engine operating conditions to control the fuel injection rate.
In this type of electronic fuel injection system, the fuel ignition timing is an important factor having great influence on the engine performance. Usually, the fuel injection timing is detected by a pulser coil or similar detection means. The pulser coil is so disposed that it faces a magnet provided on the outer periphery of a flywheel connected to a crankshaft, for instance, and the fuel injection timing is obtained according to a magnet detection signal, i.e., a crankshaft rotation detection signal, from the pulser coil. FIG. 6 shows a timing example of the pulser voltage, the shaped waveform obtained therefrom and the fuel injection valve energizing signal.
In a capacitor discharge ignition (CDI) system, the pulser signal for forming the ignition timing is also used for deciding the fuel injection timing.
A self-triggered ignition system, which does not require any pulser signal for fuel injection timing formation, requires an exclusive pulser coil for the fuel injection timing detection. Recently, however, researches and investigations have been conducted concerning fuel injection timing detection systems without any exclusive pulser. As an example, a system has been suggested for obtaining a fuel injection timing signal based on a primary coil voltage waveform in a self-triggered ignition system.
FIG. 7 is a timing chart illustrating this method. It was thought to use the positive or negative part of the voltage induced in the primary coil as a signal source. It has been noted that, however, if it is attempted to obtain a fuel injection timing signal from the negative voltage part used as an energy source for ignition, the energy assigned for the ignition is greatly attenuated. Therefore, it is impossible to obtain a fuel injection timing signal by this method.
Accordingly, studies have been conducted to detect the rising of the positive voltage part of the shaped waveform, and to obtain a fuel ignition timing on the basis of the detected waveform rising. The waveform rising is detected twice in one fuel ignition cycle, and one of the two detections is used as the fuel injection timing. Referring to FIG. 7, a fuel injection valve energizing signal A is produced on the basis of the first appearance of the two waveform risings detected in one cycle, and another fuel injection valve energizing signal B is obtained on the basis of the second waveform rising. In this case, a separate sensor is necessary for judging which one of the fuel injection valve start signals A and B is to be used. In addition, noise is likely to be introduced into the primary coil voltage waveform, making the waveform judgment more difficult.
Recently, there has been an attempt to apply the electronically controlled fuel injection system to small displacement engines such as universal engines and agricultural engines which are not provided with any power source battery, and which are manually started with rope starter or recoil starter systems (Japanese Patent Laid-Open No. 4-43843). In such small displacement engines, the electric power obtainable from a flywheel generator is low during a cranking period, and therefore it is a very important problem to secure a source voltage that is necessary for starting the engine. This means that power for ignition has to be obtained by taking it into consideration to avoid adverse effects on a microcomputer and fuel injection valve drive system.
In the above background, the self-triggered ignition system is desirably used for a small displacement engine, and there is a demand for realizing a practical system for obtaining fuel injection timing according to a signal obtained in the self-triggered ignition system.